The present invention relates to a process for producing a diaryl carbonate. More particularly, the present invention relates to a process for producing a diaryl carbonate according to the disproportionation reaction of an alkylaryl carbonate.
It has been already known to obtain diphenyl carbonate by disproportionation of an alkylphenyl carbonate [Japanese Patent Publication No. 48537/1983 (U.S. Pat. No. 4,045,464)]. However, in this method, a catalyst selected from among Lewis acids and transition metal compounds capable of forming Lewis acids is used, and specifically AlX.sub.3, TiX.sub.3, UX.sub.4, VOX.sub.3, VX.sub.5, ZnX.sub.2, FeX.sub.3, SnX.sub.4 are included, wherein X is a halogen, an acetoxy group, an alkoxy group, or an aryloxy group. Such Lewis acids are highly corrosive to the metallic materials of reaction vessels, pipelines and valves. Thus methods using these Lewis acids involve a problem when industrially practiced.
Further, it has been also proposed to conduct such a method using a catalyst consisting of a mixture of a Lewis acid and a protonic acid [Japanese Laid-open Patent Application No. 173016/1985 (U.S. Pat. No. 4,609,501)]. However, in this method, because a protonic acid is used in addition to a Lewis acid, not only the problem of corrosion becomes more serious, but also separation and recovery of the catalyst are difficult.
Also, some proposals have been made to use organic tin compounds or organic titanium compounds as a catalyst in such methods [Japanese Laid-open Patent Publications Nos. 169444/1985 (U.S. Pat. No. 4,554,110), 169445/1985 (U.S. Pat. No. 4,552,704), 265062/1989, 265063/1989), but these organic tin compounds or organic titanium compounds are difficult to separate completely from the products whereby it is very difficult to obtain the diaryl carbonate with high purity. One important use of a diaryl carbonate is its use as a monomer for producing polycarbonate, and in this case, if an organic tin compound or an organic titanium compound is present, even in a small amount, in the diaryl carbonate, the physical properties of the polycarbonate thus produced will be lowered. Therefore, the diaryl carbonate produced by the methods using such catalysts cannot be effectively used as the monomer for producing the polycarbonate.
Further, it has been also proposed to use in such a method a catalyst selected from a compound containing Sc, Cr, Mo, W, Mn, Au, Ga, In, Bi, Te, and lanthanoid (Japanese Laid-Open Patent Application No. 265064/1989). However the yield of the desired diaryl carbonate is low in this method.